The invention relates to a method of and an arrangement for determining the degree of sharpness of an original, e.g., an exposed and developed negative. A determination of the degree of sharpness may be of importance, for example, in making a decision as to whether reproduction of the original is worthwhile.
The West German Offenlegungsschrift No. 26 54 943, and the corresponding, commonly-assigned U.S. Pat. No. 4,379,632, disclose a method of distinguishing between sharp and fuzzy originals. An original is photoelectrically scanned line-by-line in such a manner that the width of each linear, scanned region equals or approximates the size of the smallest visually observable detail of the original. A scanning signal is generated and represents the density of the original. The scanning signal is divided into two parts, and one of these parts is filtered to eliminate the high frequency range thereof so that it resembles a signal derived from a fuzzy original. In this manner, the initial scanning signal is processed to yield a first signal having a relatively large bandwidth, and a second signal having a relatively small bandwidth.
Each of the signals obtained from the initial scanning signal is differentiated, and the maximum values of the differentials are ascertained. The maximum values of the respective differentials represent the maximum density gradients of the corresponding signals. A ratio Qv is established for the respective original and equals the ratio of the maximum density gradients of the first and second signals. This ratio is compared with a reference value, and a final determination as to the sharpness of the original, or possibly only a portion of the original, is made based on the results of the comparison. The original is judged to be sharp when the magnitude of the ratio exceeds the reference value.
The above method of analyzing the scanning signals in order to determine the sharpness of an original is susceptible to error. For example, an erroneous conclusion may be reached when a dust particle having well-defined edges and a high density at its center is present on the original and is detected during the scanning operation.
Another consideration in distinguishing between sharp and fuzzy originals is that the operators of copying apparatus are very interested in avoiding the copying of fuzzy originals since this enables labor and material costs to be reduced. On the other hand, care must be taken not to classify borderline originals which can still yield relatively good copies as fuzzy because this reduces the number of originals copied and may lead to customer complaints. Accordingly, efforts are made to prevent borderline originals capable of yielding relatively good copies from being classified as fuzzy under any circumstances. To this end, the examination of originals for sharpness is preferably performed in such a manner that borderline originals are classified as sharp even though this may result in improper classification of some fuzzy originals as sharp.